The present invention relates to a sheet-shaped electrode, a method of producing the same, and a secondary battery using the sheet-shaped electrode.
Since electroconductive polymeric materials such as polyacetylene, polypyrrole, and polyaniline are lighter in weight as compared with metals, possess high electroconductivity by doping anions or cations and are electrochemically active, their applications to electrochromic elements, various sensors, electrodes of batteries and the like have been studied.
Among such electroconductive polymeric materials, an aniline type polymer is considered to be a promising electrode active material for electrode because of its superior characteristics such as its preparation being comparatively easy, having high stability against oxidation by air, and against water, and its oxidation-reduction being electrochemically reversible, having a long cycle life, and a small self-discharge.
However, when the aniline type polymer itself is used as the foregoing sheet-shaped electrode, it does not function adequately as collector. Therefore it is desirable to combine the aniline type polymer with other collectors when using the polymer as the electrode, in consideration of the improvement of the internal impedance and the reliability thereof rather than using the polymer alone.
Furthermore, an advantage of polymeric active materials such as aniline type polymer is that they can be worked into a sheet. When using such a polymeric active material, it is preferable that the collector for holding the same be also of a sheet type.
Particularly, when it is applied to a battery, it is preferable that the collector be light in weight and not voluminous with respect to the energy density thereof.
From the view point of the reduction in weight and thin filming of the collector, application of aluminum to the collector can be conceived. However, the surface of aluminum is generally covered with an aluminum oxide film, and the oxide film hinders the formation of a uniform aniline type polymer film on an aluminum substrate by the electrochemical polymerization of an aniline type monomer.
In order to solve this problem, a proposal is made in Japanese Laid-Open Patent Application No. 61-133557, in which it is disclosed that an electroconductive coating layer of a metal or carbon is formed on an aluminum substrate on which a naturally oxidized overcoat layer remains, and alternatively, an electroconductive polymer film is formed by electrochemical polymerization on an aluminum substrate by the steps of removing a naturally oxidized overcoat layer from the surface of the aluminum substrate by cleaning the same with a caustic alkali to expose its brightened surface, and then forming the electroconductive polymer film directly on the brightening surface of the aluminum substrate, or after forming a heavy metal coating layer thereon.
However, in the above case, even if direct electrochemical polymerization is tried on the brightening surface of the aluminum substrate from which the oxidized layer has been removed, the aluminum dissolves into the reaction solution of electrochemical polymerization so considerably that it is extremely difficult to form an aniline type polymer film on the brightening aluminum surface. Further even if the electrochemical polymerization is carried out in a selected reaction solution in which the aluminum is slightly dissolved, it is still difficult to form a polymer film on the aluminum substrate. Also, the provision of the electroconductive coating or a heavy metal coating complicates the process for the production of the electrode, reduces its productivity, and causes new problems such as poor adhesion between the aluminum collector and the aniline type polymer, and unreliable performance of the battery.
Such adhesion properties between the active material for the electrode and the collector is so important for increasing the collection efficiency that many studies have been made on the collectors for use in organic secondary batteries. For example, in Japanese Laid-Open Patent Application Nos. 58-115776 and 58-115777, there are disclosed methods of making collectors adhere to polymeric active materials. These methods, however, have the shortcomings that the film forming properties and mechanical strength of the polymeric active materials are poor, and the close contact between the collectors and the polymeric active material is not so completely achieved that the polymeric active material easily peels and falls off the collector during repeated charging and discharging, resulting in poor collection efficiency and reduction of its cycle life.
Furthermore, Japanese Laid-Open Patent Application Nos. 62-20243 and 61-133557 disclose an electrochemical polymerization for producing electroconductive polymers by use of a collector, such as an expanded metal and a metal network, in an electrode. When such an expanded metal is used, the morphological characteristics of conductive polymers are not fully utilized, and the control of their morphology is difficult. Possibly this is because the morphology of each conductive polymer is determined by the diameter and shape of the wire-like material. Also, these two-dimensional network type metals are difficult to work into a thin sheet, particularly into a sheet with a thickness of 50 .mu.m or less, so that they are not suitable for use as a collector of a sheet-like electrode for a thin type battery. Furthermore, since the electroconductive polymers are formed around each wire material of the two-dimensional network structure, it is difficult to grow the polymers between the gratings of the network and, even if such growth takes place, the electroconductive polymer on the wire materials has such a considerable thickness that the uniformity in the thickness of the electrode is practically lost.